The present invention relates to a semiconductive polymer composition having easily controllable stripping characteristics, especially for an electric power cable. The present invention further relates to the use of such a semiconductive polymer composition and an electric power cable comprising at least one layer comprising said semiconductive polymer composition.
Electric power cables for medium to high voltages normally include one or more metal conductors surrounded by an insulating material like a polymer material such as an ethylene polymer.
In power cables, the electric conductor is usually coated first with an inner semiconducting layer, followed by an insulating layer, then an outer semiconducting layer, followed by optional layers such as water-barrier layers and on the outside optionally a sheath layer. The layers of the cable are commonly based on different types of ethylene polymers.
The insulating layer and the semiconducting layers normally consist of ethylene homo- and/or copolymers which are preferably crosslinked. LDPE (low density polyethylene, i.e. polyethylene prepared by radical polymerization at a high pressure) crosslinked by adding peroxide, e.g. dicumyl peroxide, in connection with the extrusion of the cable, has recently become the predominant cable insulating material. The inner semiconducting layer normally comprises an ethylene copolymer, such as an ethylene-vinyl acetate copolymer (EVA), ethylene methylacrylate copolymer (EMA), ethylene ethylacrylate copolymers (EEA), ethylene butylacrylate copolymer (EBA). The composition of the outer semiconducting layer differs depending on whether it has to be strippable or not.
A common concept for making an semiconductive layer strippable from an usually non-polar insulation layer is to increase the polarity of the semiconductive layer. This is e.g. done by the addition of highly polar acrylonitrile-butadiene rubber (NBR) to the semiconductive composition which e.g. further comprises an ethylene copolymer, such as an ethylene-vinyl acetate copolymer (EVA) and sufficient carbon black to make the composition semiconducting.
As an example of a strippable composition, mention may be made of EP-B1-0 420 271 which discloses a semiconducting insulation shielding composition for electric cables which, based on the total weight of the composition, consists essentially of (A) 40-64% by weight of an ethylene-vinyl acetate copolymer with 27-45% of vinyl acetate, (B) 5-30% by weight of an acrylonitrile-butadiene copolymer with 25-55% of acrylonitrile, (C) 25-45% by weight of carbon black having a surface area of 30-60 m2/g, and (D) 0.2-5% by weight of an organic peroxide crosslinking agent. In addition, the composition may include 0.05-3% by weight of conventional additives.
As a further example of prior art, strippable semiconducting compositions for electric cables, mention may be made of U.S. Pat. No. 4,286,023 which discloses a polymer composition for electric cables comprising (A) an ethylene copolymer selected from the group consisting of ethylene-alkyl acrylate copolymers containing about 15-45% by weight of alkyl acrylate, said alkyl acrylate being selected from the group consisting of C1-C8 alkyl esters of (meth)acrylic acid, such as methyl acrylate, ethyl acrylate, methyl methacrylate, butyl acrylate, 2-ethyl-hexyl acrylate and the like, and ethylene-vinyl acetate copolymers containing about 15-45% by weight of vinyl acetate, (B) a butadiene-acrylonitrile copolymer (nitrile rubber) containing about 10-50% by weight of acrylonitrile (C) conductive carbon black, and (D) a peroxide crosslinking agent, wherein the weight ratio A:B=1:9 to 9:1; C:(A+B)=0.1 to 1.5, and D is present in an amount of 0.2-5% by weight of the total composition.
Although prior art compositions for semiconducting layers in electric cables are satisfactory for many applications, there is always a desire to improve their characteristics and eliminate or reduce any disadvantages they may have.
Furthermore, WO 99/20690 discloses an inner semiconducting composition for electric cables which, based on the total weight of the composition, comprises                (a) 30-90% by weight of an ethylene copolymer,        (b) carbon black to make the composition semiconducting,        (c) 0-8% by weight of a peroxide crosslinking agent,        (d) 0-8% by weight of conventional additives,wherein the ethylene copolymer (a) is an ethylene-methyl(meth)acrylate copolymer. It is reported that the use of EMA improves the thermostability of the polar copolymer and the composition containing the same. Thus, the composition can be heated to higher temperatures, e.g. during compounding and crosslinking in other known compositions. Consequently, a higher production rate during compounding and a higher line speed is possible in cable production.        